Acoustic properties of fracture of dental restorative materials and endocrown restorations under quasi-static loading.

OBJECTIVES: (1) To investigate the properties of acoustic emission (AE) signals from fracture of dental materials and endocrown restorations during quasi-static compression using the continuous wavelet transform analysis. (2) To determine by the parameters of AE signals in which structural element of tooth-endocrown system the fracture starts. METHODS: Five restorative materials (zirconium dioxide (Prettau zirconia), ceramics (IPS e.max Press), metal ceramics (GC Initial MC+Nicrallium N2 BCS), composite resin (Nano Q), luting cement (Relyx U200)) were manufactured into discs, 13mm diameter, 5mm thick, which were divided into 5 groups (Group B, C, D, E, F, respectively) and included into Type 1. Twenty-five extracted human molars were divided into 5 groups and included into Type 2: Group A (control, no restoration); BE (tooth restored by zirconium dioxide endocrowns); CE (restored by ceramic endocrowns); DE (restored by metal ceramic endocrowns); EE (restored by composite resin endocrowns). An increasing load was applied to the center of the samples with a hard steel ball until a fracture occurred. The loading rate was 0.12mm/min. Two-channel AE detection system was used to record the AE activity during testing. The parameters including the peak frequency, bandwidth, duration time and energy of AE signals were analyzed. RESULTS: The restorative materials (Type 1) differ from each other by acoustic properties such as the peak frequency main bands and energy of AE signals. The detected AE signals correspond to the different fracture types. The dominant fracture mechanism of Group C, E and F specimens was the macrocracking (brittle fracture), and of Group D specimens was the microcracking, which alternated with the plastic deformation and macrocracking. For the number of events corresponding to the brittle fracture, statistically significant differences (p<0.05) were found between all group pairs. AE signals were not released during compression test of the zirconium dioxide samples, thus the fracture was not detected. All the AE signals detected during compression test of the different endocrown systems (Type 2) had two peak frequency bands. The fracture of Groups A, BE, CE and DE samples characterized by alternating the micro- and macrocracking with plastic deformation, and the brittle fracture was the dominant fracture mechanism. The restorations of Group EE fractured without plastic deformation. For the number of events corresponding to the brittle fracture, statistically insignificant differences (p>0.05) were found between Groups A vs CE and A vs EE, and statistically significant differences (p<0.05) were found between Groups A vs BE and A vs DE. SIGNIFICANCE: The initiation of fracture in the restorations with endocrowns made from different materials begins with the tooth damage. The metal ceramic endocrown restoration showed the highest fracture resistance, what is especially important for clinical use.